<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            width: 100px;
            height: 100px;
        }
    </style>
</head>

<body>

    <div></div>
</body>
<script src="js/jquery.min.js"></script>
<script>
    $(() => {
        $('div:first')
            .mouseover(() => {
                $('div:first').css('backgroundColor', 'blue');
            })
            .click(() => {
                alert('我是单击');
            })

        .css('backgroundColor', 'red');

    })
</script>

</html>